In recent years, various mobile communication apparatuses utilizing radio waves have been developed. While most of the mobile communication apparatuses have been put to practical use, RFID (Radio Frequency Identification) systems using radio IC tags attract much attention among these apparatuses. The radio IC tag used in the RFID system includes a module substrate with an antenna having a radiation conductor, and a semiconductor IC mounted on the module substrate (see Japanese Patent Application Laid-open No. 2003-298464). By incorporating this radio IC tag into a product tag, a prepaid card, or the like, conveniences of physical distribution management and customer management, as well as billing and settlement are expected to be greatly improved.
Since the radio IC tag is desirably made as small as possible in view of its specific use, the module substrate with an antenna needs to be miniaturized. However, if the module substrate with an antenna is miniaturized, the area of the radiation conductors (antenna conductors) provided on the module substrate with an antenna is reduced. As a result, the antenna band is narrowed and gain is reduced.
In view of this, various methods of miniaturizing the radiation conductors while maintaining desirable characteristics, have been proposed. For example, Japanese Patent Application Laid-open No. 2002-135037 discloses a method of miniaturizing the radiation conductor in which two sides of each radiation conductor constituting a so-called bowtie antenna are inwardly concaved in an arc shape or a stepped shape.
However, accompanying an increasing demand for further miniaturization of the radio IC tags, it is difficult to sufficiently miniaturize the module substrate with an antenna by the method disclosed in Japanese Patent Application Laid-open No. 2002-135037. Miniaturization of the module substrates with an antenna is of course important even if their application field is other than the radio IC tags.